This fixes a possible circular locking dependency detected warning seen
with:
- CONFIG_PROVE_LOCKING=y
- consumer/provider IIO devices (ex: "voltage-divider" consumer of "adc")
When using the IIO consumer interface, e.g. iio_channel_get(), the consumer
device will likely call iio_read_channel_raw() or similar that rely on
'info_exist_lock' mutex.
typically:
...
mutex_lock(&chan->indio_dev->info_exist_lock);
if (chan->indio_dev->info == NULL) {
ret = -ENODEV;
goto err_unlock;
}
ret = do_some_ops()
err_unlock:
mutex_unlock(&chan->indio_dev->info_exist_lock);
return ret;
...
Same mutex is also hold in iio_device_unregister().
The following deadlock warning happens when:
- the consumer device has called an API like iio_read_channel_raw()
at least once.
- the consumer driver is unregistered, removed (unbind from sysfs)
======================================================
WARNING: possible circular locking dependency detected
4.19.24 #577 Not tainted
------------------------------------------------------
sh/372 is trying to acquire lock:
(kn->count#30){++++}, at: kernfs_remove_by_name_ns+0x3c/0x84
but task is already holding lock:
(&dev->info_exist_lock){+.+.}, at: iio_device_unregister+0x18/0x60
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #1 (&dev->info_exist_lock){+.+.}:
__mutex_lock+0x70/0xa3c
mutex_lock_nested+0x1c/0x24
iio_read_channel_raw+0x1c/0x60
iio_read_channel_info+0xa8/0xb0
dev_attr_show+0x1c/0x48
sysfs_kf_seq_show+0x84/0xec
seq_read+0x154/0x528
__vfs_read+0x2c/0x15c
vfs_read+0x8c/0x110
ksys_read+0x4c/0xac
ret_fast_syscall+0x0/0x28
0xbedefb60
-> #0 (kn->count#30){++++}:
lock_acquire+0xd8/0x268
__kernfs_remove+0x288/0x374
kernfs_remove_by_name_ns+0x3c/0x84
remove_files+0x34/0x78
sysfs_remove_group+0x40/0x9c
sysfs_remove_groups+0x24/0x34
device_remove_attrs+0x38/0x64
device_del+0x11c/0x360
cdev_device_del+0x14/0x2c
iio_device_unregister+0x24/0x60
release_nodes+0x1bc/0x200
device_release_driver_internal+0x1a0/0x230
unbind_store+0x80/0x130
kernfs_fop_write+0x100/0x1e4
__vfs_write+0x2c/0x160
vfs_write+0xa4/0x17c
ksys_write+0x4c/0xac
ret_fast_syscall+0x0/0x28
0xbe906840
other info that might help us debug this:
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock(&dev->info_exist_lock);
lock(kn->count#30);
lock(&dev->info_exist_lock);
lock(kn->count#30);
*** DEADLOCK ***
...
cdev_device_del() can be called without holding the lock. It should be safe
as info_exist_lock prevents kernelspace consumers to use the exported
routines during/after provider removal. cdev_device_del() is for userspace.
Help to reproduce:
See example: Documentation/devicetree/bindings/iio/afe/voltage-divider.txt
sysv {
compatible = "voltage-divider";
io-channels = <&adc 0>;
output-ohms = <22>;
full-ohms = <222>;
};
First, go to iio:deviceX for the "voltage-divider", do one read:
$ cd /sys/bus/iio/devices/iio:deviceX
$ cat in_voltage0_raw
Then, unbind the consumer driver. It triggers above deadlock warning.
$ cd /sys/bus/platform/drivers/iio-rescale/
$ echo sysv > unbind
Note I don't actually expect stable will pick this up all the
way back into IIO being in staging, but if's probably valid that
far back.
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Fixes: ac917a8111 ("staging:iio:core set the iio_dev.info pointer to null on unregister")
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
7f75591fc5